As one of the control methods of internal combustion engines, a method that determines a manipulated variable of each actuator with an efficiency and an air-fuel ratio as well as a torque as controlled variables is known, as disclosed in, for example, Japanese Patent Laid-Open No. 2010-53705 and Japanese Patent Laid-Open No. 2009-047101. The efficiency mentioned here means a ratio of a torque that is actually outputted to a torque that an internal combustion engine can potentially output. Since an amount of heat that is supplied to an exhaust gas purifying catalyst via exhaust gas changes in accordance with an efficiency, the temperature of the exhaust gas purifying catalyst can be controlled by controlling the efficiency. In the arts disclosed in the above Patent Literatures, the requested efficiencies are directly used to calculate throttle openings. More specifically, a target air amount is calculated based on a value obtained by dividing a requested torque by a requested efficiency, and a throttle opening is determined based on the target air amount. Subsequently, an estimated value of the torque that is realized in the throttle opening is calculated with optimum ignition timing as a prerequisite, and with the ratio of the requested torque to the estimated value set as a target efficiency, ignition timing is determined. The respective actuators are operated in accordance with the throttle opening and the ignition timing determined in this manner, whereby the efficiency as is requested is realized as a result.
Incidentally, ordinary internal combustion engines for vehicles include multi-cylinder internal combustion engines each having a number of cylinders. In some of multi-cylinder internal combustion engines, cylinders are divided into a plurality of groups, and an exhaust gas purifying catalyst is installed for each of the plurality of cylinder groups. For example, in the V-internal combustion engine described in Japanese Patent Laid-Open No. 2010-019227, an exhaust gas purifying catalyst is installed in each of a left and right bank. In such an internal combustion engine, the efficiency is desirably controlled on a cylinder group-by-cylinder group basis from the viewpoint of exhaust gas performance. This is because the shapes of exhaust pipes differ by cylinder group, and there is a difference in warming-up performance of the exhaust gas purifying catalyst between the cylinder groups.
However, the requested efficiency that can be realized by the aforementioned control method is only one value, that is, the requested efficiency as an entire internal combustion engine. This is because, in the aforementioned control method, control of the efficiency by ignition timing is performed as part of a set with control of the air amount by the throttle opening. In the case of an internal combustion engine including a single throttle, the number of values of the throttle opening that is determined is one, and therefore, the number of values of the efficiency that can be realized in correspondence with this is also one. Accordingly, in the internal combustion engine in which a throttle is provided for each cylinder group, the throttle opening can be controlled on a cylinder group-by-cylinder group basis, and therefore, a different efficiency can be realized for each cylinder group. However, in many ordinary multi-cylinder internal combustion engines, a single throttle is shared by a plurality of cylinder groups. Even if the aforementioned control method is applied to the internal combustion engines like this, it is not easy to realize a different efficiency at each cylinder group.
Furthermore, in the case of a multi-cylinder internal combustion engine having an exhaust gas purifying catalyst for each cylinder group, air-fuel ratio feedback control is performed for each cylinder group. Therefore, in such an internal combustion engine, a different value is requested at each cylinder group with respect to not only the efficiency but also the air-fuel ratio. However, Japanese Patent Laid-Open No. 2010-53705 and Japanese Patent Laid-Open No. 2009-047101 mentioned above do not describe the control method in the case of performing operation with a different air-fuel ratio at each cylinder group.